Insulating Flat Roof (and moisture)

[Mark-J]

I'm about to install Kingspan (TR26) insulation panels between joists under a flat roof shed. The panels will be propped up and covered with plywood boards. It's a 6m x 5m roof with fibreglass on the exterior.

I am going to leave 2" gaps around the edges, to help with ventilation to let everything breath and to help with air circulation.

I apologise for my attempt at a drawing :)



Is there anything else I need to do to prevent my tools from rusting, is a 2" gap big enough?

[Mark-J]

This pic has a gap above the Kingpsan and below it. Would this be better?

[Mark-J]

Also, do I need some, breathable membrane with that setup?

The Kingspan is 100mm thick and the joists are 170mm deep so there's a gap above and below the Kingspan of 35mm (so about 1.4")

[cotswold builders]

Its a cold roof construction, either ram it tight with insulation, no gaps, then vapour barrier and board.

If its vented through, leave 50mm air gap, insulation v/b and board.

[Mark-J]

Thanks all for the advice. I've just painted the floor with resin and I'm about to start on insulating the roof. Which of the following three configurations should I go for?



Also, is it still advisable to use a breathable membrane? I'll be inserting vents either side of the joists to facilitate airflow if it's important to the breathable membrane question.

[cotswold builders]

Construct as C then vapour barrier under joists before boarding

[Mark-J]

Thanks CB.

When you say under joists, do you mean, literally affix the v/b under the joists?

Do you mean like this:



Or this: